In their thousands they came to the Garden. The ticketless loitering outside praying for luck; inside the fortunate ones hurrying for their seats, the privileged few allowed within the hallowed enclosure by the stage. Young girls in cropped tops, middle-aged men in non-ironic mullets, balding bankers and struggling students, Kofi Annan and rightwing pundit Bill O'Reilly, Julia Roberts and the Strokes, they all came to the Garden, to see a group they could all agree upon.

It is hard to imagine a time when U2 were not colossal. Musical fashions come and go like Tory party leaders but U2 seem to have been always with us. When they released their first album, 25 years ago this October, Jimmy Carter was President and John Lennon was still alive.

A quarter of a century later and U2 have long stopped being merely a rock band. In Bono they have a man who can legitimately claim to have changed the world through his work with debt relief. It is tempting to believe their success to be inevitable. But as Jim Kerr might confirm, if you could find him, longevity in the music industry is never inevitable.

They walk on, one by one. Adam Clayton, Larry Mullen Junior, Edge and finally Bono, dressed in black with inevitable sunglasses. A nervous energy hovers over them. It is evident from their faces and the way they take their positions how important New York is to U2. The band first played here in December 1980 and since then New York has become second only to Dublin in the band's affections.

Their last album contained a track named after the city and their latest album features a song that recalls the magical memory of that first visit. And it is with 'City of Blinding Lights' that U2 open their only New York concert of the summer. Bono prowls across the stage and along an ellipse-shaped catwalk which stretches into the arena. Behind Edge, Adam and Larry a curtain of lights glows. The lights are understated but hugely effective; during later songs they act like a huge pixelated screen for animations and images.

'Vertigo' and 'Elevation' arrive like a double barrelled onslaught of chords and choruses against which resistance is futile. Edge assaults his guitar with unassuming artistry, Adam smiles beatifically and Larry pounds mercilessly. Bono, shades still firmly in place, sings at the far end of the ellipse and with each step towards the frenzied crowd, another sea of hands rises each one holding a camera phone.

'Electric Co' and 'An Cat Dubh', neither played regularly for two decades, are revisited, their themes of morality finding echoes in the latest album. Seven songs from How to Dismantle an Atomic Bomb follow , including a stirring 'Miracle Drug'. It is impossible not to recall that it was during the last tour that Bono was flying back after concerts to be at his dying father's side.

For most of the night Bono was the iconic rock star; the theatrical bends of the knees, the ritualistic extracting of a girl from the crowd to dance. The most powerful moments of the night however were when the sunglasses came literally and figuratively off and where one sensed that Bono was singing not for the crowd but for himself. During 'Sometimes you can't make it on your own' he appeared lost in his own world. Shorn of the armour of his sunglasses and singing, pleading 'Can you hear me when I sing, you're the reason why the opera is in me' as his voice stretched, rose and filled Madison Square Garden you saw not a rock star, but a 45-year-old man paying tribute to a father he still misses.

From there it was on to the crowd-pleasing classics. An undimmed 'Sunday Bloody Sunday', 'Pride (in the name of love)' and a towering 'Where the streets have no name'. All greeted rapturously but what was far more compelling was an incendiary 'Bullet the Blue Sky', where Bono, wearing a bandana with symbols representing Islam, Christianity and Judaism, pointed at his temple saying: 'This city is the best evidence in the world that all three can co-exist.'

With the light-curtain lit to display the flags of the world, Bono urged the United States to lead the fight against extreme poverty. 'When America leads, the world follows,' he declared, 'and now a new challenge is here not to put man on the Moon but to put mankind back on Earth and to bring equality to the people of Africa.' The screens above the stage displayed a telephone number to text support for their campaign.

The second spine-chilling moment occurs during 'One', which is dedicated to the economist and author Jeffrey Sachs. The song remains U2's finest moment: a hymn for godless times. As the band plays, the stage lights are dimmed and Madison Square Garden is illuminated by thousands of mobiles held in the air. Like the raising of lighter flames, the call to a friend during a particular song has become a rock concert cliché.

But tonight that cliché is transformed into something moving. The starry night appears to have fallen from the sky on to Madison Square Garden as thousands in the building and thousands more in bars, apartments, college dormitories and who knows where else sing along to a song about unity, community and the fragility of both.

The encores see the return of 'Zoo Station', 'Mysterious Ways' and 'The Fly', as well as an unexpected 'Jean Genie' for David Bowie, who was in attendance, and a final blast of an encore performance of 'Vertigo'. They walk off stage secure that New York remains theirs.